1. Field of the Invention
The present invention relates to a pixel circuit of a liquid crystal display device, and, more particularly, to a pixel circuit of a multi-domain vertical alignment liquid crystal display device.
2. Description of Related Art
Currently, a mode most widely adopted for the TFT LCD is a normally-white mode that is implemented in a twisted nematic (TN) LCD. The technology of manufacturing the TN TFT LCD has outstandingly advanced in recent years. Contrast and color reproducibility provided by the TN TFT LCD have surpassed those offered by the CRT
The color saturation of the TN-LCD device is better than that of the conventional display devices (e.g. the CRT display devices). However, despite the related TN technologies improve a lot recently, the disadvantage of the TN-LCD devices such as narrow viewing angle still needs to be improved. Moreover, the application of the TN-LCD device is also limited by the disadvantage of the narrow viewing angle.
On the other hand, as the liquid crystal molecules are driven in a VA mode, the liquid crystal molecules do not be rotated without applying electrical field, The light from the backlight module won't pass through the liquid crystal layer and the polarizer, and then display a black screen, i.e. the “normally black”.
Generally, the contrast of the VA-LCD is higher than that of the TN-LCD. The response speed of the VA-LCD is faster than that of the TN-LCD. Furthermore, the viewing angle of the VA-LCD is wider than that of the TN-LCD. Hence, VA-LCD attracts people's attraction recently.
However, the distribution of the brightness of the VA-LCD is not homogeneous, because each liquid crystal molecule has a character of the birefringence.
FIG. 1 is the schematic view of a pixel 1 of a conventional multi-domain vertical alignment (MVA) liquid crystal display device. The conventional MVA-LCD device comprises a first substrate 11 and a second substrate 12. A liquid crystal layer with a plural of liquid crystal molecules 141, 142, 143, 144 is interposed between the first substrate 11 and the second substrate 12. A top electrode 111 is formed on the surface of the first substrate 11, and moreover, several protrusions 13 are formed on the surface of the first substrate 11. On the other hand, several pixel electrodes 121 and plural slits 15 are formed on the surface of the second substrate 12. Each slit 15 is located between the pixel electrodes 121.
Since the average tilted angle of the whole pixels of the conventional MVA LCD of the same gray level is identical, the tilted angle of the liquid crystal molecules of a sub-pixel is identical. In other words, the tilted angle of the liquid crystal molecules 141, 142 is as same as that of the liquid crystal molecules 143, 144 in FIG. 1.
FIG. 2 is schematic view of a pixel of a conventional MVA LCD. The circuit of the pixel 2 of the conventional MVA LCD comprises two sub-pixels 21, 22. The sub-pixel 21 further comprises a thin film transistor 211, a storage capacitor (Cs1) 212, a common voltage 213, and a liquid crystal capacitor (Clc) 214. The sub-pixel 22 further comprises a thin film transistor 221, a storage capacitor (Cs2) 222, a common voltage 223, and a liquid crystal capacitor (Clc) 224.
The common voltages (Vcom) 213, 223 is provided by a storage bus line, where the provided common voltage is a constant. The liquid crystal capacitor 214, 224 comprises a top electrode, a pixel electrode, and a liquid crystal layer interposed between the top electrode and the pixel electrode. The pixel electrode provides the operational voltage for operation. Hence, by controlling the difference between the operational voltage and the common voltage, the electrical field of the liquid crystal layer, or even the rotation of the liquid crystal molecules can be controlled.
Since the common voltages applied on one terminal of the storage capacitor 212, 222 are identical, and the capacitances of the liquid crystal capacitor 214, 224 are almost the same, the tilted angles of the liquid crystal molecules of the sub-pixels 21, 22 are almost the same under the same gray level. Hence, the distribution of the brightness of the conventional multi-domain vertical alignment liquid crystal display device is not homogeneous. The display quality of the conventional multi-domain vertical alignment liquid crystal display device deteriorates, too.
To overcome the problem illustrated above, Japan Patent Application Publications Nos. JP2004-258139 and JP2004-62146 disclosed an LCD device in which each pixel is divided into two sub-pixels by two pixel electrodes and two storage capacitors. The first and the second operation voltages V1(gk) and V2(gk) are applied to each pixel. The difference between the voltages V1(gk) and V2(gk) is determined by a capacitance-coupling driving. Accordingly, JP2004-258139 and JP2004-62146 use the divided sub-pixels and the different operation voltages to improve the angular Gamma offset of partial gray-tone. FIGS. 11 to 13 are schematic graphs of Gamma curves, as shown in JP2004-258139 and JP2004-62146, where a Y-axis indicates a voltage V and transmittance T and an X-axis indicates a gray-tone transmittance. As shown in FIG. 11, the angular Gamma offset is improved on the low gray-tone as the pixel ratio of a first to a second sub-pixel is 30:70, where the first sub-pixel has 30% area of the pixel, and the second sub-pixel has 70% area of the pixel. As shown in FIG. 12, the angular Gamma offset is improved on the low gray-tone as the pixel ratio of a first to a second sub-pixel is 70:30, where the first sub-pixel has 70% area of the pixel, and the second sub-pixel has 30% area of the pixel. As shown in FIG. 13, the angular Gamma offset is improved on the low gray-tone as the pixel ratio of a first to a second sub-pixel is 50:50, where the first sub-pixel has 50% area of the pixel, and the second sub-pixel has 50% area of the pixel. However, in the above-mentioned Japan Patent Application Publications, the angular Gamma offset of all gray-tone still cannot be proved effectively. Therefore, the MVA LCD provided by the above-mentioned Japan Patent Application Publications still encounter the problem of the non-homogeneous distribution of the brightness. Therefore, it is desirable to provide an improved method to mitigate the aforementioned problems.